Can I use a texture as hair density map?
In Cycles select the mesh, give it a material, then go the Particles header and add a new texture in the Textures panel.


Prepare your black and white density map. Here's mine:


Next go to the Texture header, add a new texture (Image or Movie) and check the Density checkbox in the Influence panel. Note: I also checked the Lenght checkbox to make the render look more clear.



In Blender Internal you can do it the same way. After giving the mesh a material and the particle system you go to the Texture header-->Particle Texture button, load your texture as Image or Movie and select what kind of influence on particles it should have.